1. Field of the Invention
The subject invention relates to an assembly of a syringe barrel, a needle cannula and a safety shield. The safety shield is telescoped over the syringe barrel and is movable between a proximal position where a needle cannula is exposed for use and a distal position where the needle cannula is shielded.
2. Description of the Prior Art
A prior art hypodermic syringe includes an elongate syringe barrel with a proximal end, a distal end and a tubular side wall extending therebetween. The side wall defines a cylindrical fluid receiving chamber within the syringe barrel. The proximal end of the syringe barrel is opened into the fluid receiving chamber. An elongated tip is defined at the distal end of the syringe barrel and has a narrow fluid passage that communicates with the fluid receiving chamber.
The prior art hypodermic syringe also includes a needle cannula with a proximal end, a distal end which is usually sharpened and a lumen extending therethrough. The needle cannula is mounted or mountable to the tip of the syringe barrel such that the lumen through the needle cannula communicates with the fluid receiving chamber in the syringe barrel. Some prior art hypodermic syringes have the proximal end of the needle cannula fixedly and permanently mounted to the tip of the syringe barrel. However, other prior art hypodermic syringes include a needle cannula that is removable mounted to the tip of the syringe barrel. For example, many prior art syringe barrels include an internally threaded luer-type collar surrounding the tip. The proximal end of the needle cannula is mounted to a needle hub which is threadedly engageable with the luer collar and which telescopes over the tip to place the lumen of the prior art needle cannula in communication with the fluid receiving chamber of the prior art syringe barrel.
The prior art hypodermic syringe further includes a plunger slidably disposed within the fluid receiving chamber of the syringe barrel. Distal movement of the plunger in the syringe barrel urges fluid from the chamber and through the needle cannula. Conversely, proximal movement of the plunger in the syringe barrel draws fluid through the needle cannula and into the fluid receiving chamber.
Most prior art hypodermic syringes include structure for preventing accidental sticks. For example, most prior art hypodermic syringes have a rigid cap mounted over the needle cannula prior to use. The rigid cap includes an open proximal end that is removably mounted to either the hub of the needle cannula or to a distal portion of the syringe barrel. The cap is removed immediately prior to use of the hypodermic syringe to expose the needle cannula. However, reshielding a used needle cannula with a prior art cap is not recommended because a misalignment of the cap and the needle cannula during this reshielding operation can result in the accidental stick. As a result, many hypodermic syringes include a safety shield that is intended to enable reshielding.
A prior art hypodermic syringe having a safety needle shield is disclosed in U.S. Pat. No. 5,658,254. This prior art syringe includes a rigid tubular safety shield that is mounted over the syringe barrel and that is movable from a proximal position to a distal position relative to the syringe barrel. The safety shield is dimensioned such that the needle cannula is exposed when the safety shield is in its proximal position on the syringe barrel. However, the dimensions of the safety shield ensure that the needle cannula is protectively surrounded when the safety shield is in a distal position on the syringe barrel. The proximal end of the prior art safety shield has internal cross-sectional dimensions that enable frictional retention of the safety shield in the proximal position. Additionally, the proximal end of the prior art safety shield is configured to lockingly engage structure at the distal end of the syringe barrel. This locking structure is configured to prevent re-exposure of the needle cannula by sliding the safety shield proximally along the syringe barrel.
Releasable retention of the safety shield in the proximal position is necessary to prevent the needle shield from sliding distally into a position where the needle shield interferes with use of the needle cannula. However, a releasable engagement that relies entirely upon a frictional interfit is difficult in view of the close manufacturing tolerances that are required. For example, a frictional interfit that is too tight may make shielding difficult. Conversely, a frictional interfit that is too loose may cause the needle shield to inadvertently slide distally along the syringe barrel while the syringe is being used.